One megawatt is the unit of power equal to 1,000 kilowatts. MW(th) denotes the thermal power of a generating station, that is, the rate at which heat is produced (by combustion if it is a conventional thermal plant, by fission in the reactor core if it is a nuclear reactor). MW(e) denotes the electrical power output of the station and is only a fraction of the thermal power - typically about 30% for a heavy water reactor and up to 40% for a modern fossil-fuel-fired station. The latter radio - MW(e)/MW(th) - is called the thermal efficiency of the power station.

A megawatt is a million watts, sufficient power to light 10,000 100-watt bulbs, or enough electricity for around 3,000 households. The watt is a unit of power, equal to one joule per second. Sometimes the unit MWe, or "megawatt electric" is used to distinguish the electrical power produced by a plant from the larger amount of heat (MWt, or megawatt thermal) required to make that electricity, due to the inherent limitation on the efficiency of converting heat to electricity.

A unit of power, = 106 watts. MWe refers to electric output from a generator, MWt to thermal output from a reactor or heat source (eg the gross heat output of a reactor itself, typically three times the MWe figure).